1. Field of the Invention
This invention relates to vehicle position sensing systems and, more particularly, to passive position sensing systems for a levitated vehicle, such as, for example, a Magnetic Levitated Vehicle (MAGLEV). The invention also relates to a method for determining the position of a moving vehicle.
2. Background Information
There are numerous situations when it is required to use a non-contacting mechanism to measure location along a track of a moving object. One example is the problem of accurately locating the position and velocity of a magnetically levitated train car moving above a linear track. Accurate information on the location of the car in such cases would, for example, be needed in order to synchronize the drive currents of a linear synchronous motor (LSM) drive with respect to the position of the train. In such drive systems, the propulsion is obtained by exciting currents in multi-phased windings that are embedded in the track. These currents interact with the magnetic fields arising from an array of permanent magnets on the moving train car. In order to use the LSM drive to accelerate the car, to keep it in motion at a constant speed, and then to decelerate it, the phase, amplitude and frequency of the currents in the LSM drive windings must be accurately controlled at all times. This requirement must be met by actively controlling the inverter that supplies the currents to the track. However, to achieve this end, it is necessary that the train car should be able to communicate its position, within an accuracy of a few millimeters, to the control circuits of the inverters. It is also desired that the communicated position should be insensitive to variations in the levitation height of the train car as might be caused, for example, by changes in the passenger loading. See, for example, United States Patent Application Publication No. 2005/0068042.
Magnetic Levitated Vehicle (MAGLEV) systems are well known in the art. Examples are disclosed in U.S. Pat. Nos. 5,517,924; 5,586,504; and 6,044,770.
Most high-speed MAGLEVs are projected to run, for example, at speeds of about 150 to about 300 mph, while low-speed MAGLEVs are projected to run at speeds of up to about 30 to about 50 mph. A wide range of different speeds is possible.
FIG. 1 shows a MAGLEV system 2 including a MAGLEV 4 and a guideway 6 having two rails 8,10. The MAGLEV 4 moves over the rails 8,10, although any suitable count of rails may be employed. Extending from the MAGLEV 4 are magnetic sources (not shown), which are configured to flank each of the rails 8,10. These rails house composite coils (not shown). As the MAGLEV 4 travels along the guideway 6, its magnetic sources extend downward, with each source flanking one of the rails 8,10 and flanking the coils housed within it.
The composite coils are incapable of levitating and stabilizing the MAGLEV 4 at low speeds. One alternative for addressing this low-speed problem is to affix wheels 12 to the bottom of the MAGLEV 4, in order to support the MAGLEV at certain speeds. The wheels 12 can be retracted as with conventional aircraft. Alternatively, the surface of the guideway 6 can be sloped away from the rail composite coil structure (not shown). Another alternative employs an additional coil (not shown) situated in the guideway 6.
Preferably, the MAGLEV 4 is “passive” and its motion is controlled by the MAGLEV guideway 6 and not by devices onboard the MAGLEV 4. For example, the guideway 6 is organized into a plurality of “zones,” such as zone 14. A control circuit 15 and an inverter, such as 16, control each zone and determine the motion of the MAGLEV 4 via the LSM drive 18. Thus, the zone inverter 16 controls the physical motion (e.g., acceleration, deceleration, speed regulation) of the MAGLEV 4, which has arrays of permanent magnets (not shown) to provide suspension and guidance forces as well as to provide the field for the LSM drive 18. Feedback-controlled currents in control coils (not shown) wound around the magnets stabilize the suspension. The LSM drive windings (not shown) are integrated with the suspension rails 8,10 and are excited by the zone inverters, such as 16, located along the guideway 6.
It is known to determine speed and position of a moving vehicle. A prior proposal for a MAGLEV inductively couples a relatively low frequency (e.g., 20 kHz) signal from the MAGLEV to a wayside loop containing a series of transpositions. The resulting analog signal is sampled and processed by a microprocessor to detect the signal peaks that correspond to the wayside loop transpositions. This proposal is extremely sensitive to in-band induced noise from the LSM drive. In addition, any slight change in vehicle height (expected to occur due to levitation) causes significant variations in signal strength measured at the wayside loop, which, in turn, alters the ability of the microprocessor to properly detect the peaks in the analog signal level.
There remains a substantial need for improvement in vehicle position sensing systems and, in particular, to such systems for a levitated vehicle, such as, for example, a MAGLEV. There is also room for improvement in methods for determining the position of a moving vehicle with respect to a guideway.